90 



TABLE 63.— APPARENT DIAMETER OF PUPIL AND FLUX DENSITY 



AT RETINA 



Flashlight measures of the pupil (both eyes open) viewed through the eye lens and 

 adapted to various field intensities. For eye accommodated to 25 cm, ratio apparent to 

 true pupil, 1.02, for the unaccommodated eye, 1.14. The pupil size varies considerably with 

 the individual. It is greater with one eye closed; e.g., it was found to be for 0.01 milli- 

 lambert, 6.7 and 7.2 mm; for 0.6 ml, 5.3 and 6.5; for 6.3 ml, 4.1 and 5.7; for 12.6 ml, 4.1 

 and 5.7 mm for both eyes and one eye open respectively for a certain individual. At the 

 extreme intensities the two values approach each other. The ratio of the extreme pupil 

 openings is about re, whereas the light intensities investigated vary over 1,000,000-fold. 



TABLE 64.— MISCELLANEOUS EYE DATA 



Light passing to the retina traverses in succession (a) front surface of the cornea 

 (curvature, 7.9 mm) ; (b) cornea (equivalent water path for energy absorption, 0.06 cm) ; 

 (c) back surface cornea (curv., 7.9 mm) ; (d) aqueous humour (equiv. H 2 0, 0.34 cm, 

 n— 1.337) ; (r) front surface lens (c, 10 mm) ; (/) lens (equiv. H 2 0, 0.42 cm, n= 1.445) ; 

 (g) back surface lens (c, 6 mm); (h) vitreous humour (equiv. H 2 0, 1.46 cm, n = 1.337). 

 An equivalent simple lens has its principal point 2.34 mm behind (a), nodal point 0.48 mm 

 in front of (g), posterior principal focus 22.73 mm behind (a), anterior principal focus 

 12.83 mm in front of (a), curvature, 5.125 mm. At the rear surface of the retina (0.15 mm 

 thick) are the rods (30 X 2/u) and cones (10 (6 outside fovea) fi long). Rods are more 

 numerous, 2 to 3 between 2 cones, over 3,000,000 cones in eye. Macula lutea, yellow spot, 

 on temporal side, 4 mm from center of retina, long axis 2 mm. Central depression, fovea 

 centralis, 0.3 mm diameter, 7000 cones alone present, 6 X 2 or 3/jl. In region of distinct 

 vision (fovea centralis) smallest angle at which two objects are seen separate is 50" to 

 70" = 3.65 to 5.14/x at retina; 50 cones in 100/u. here; 4/x between centers, 3fi to cone, 

 Ifi to interval. Distance apart for separation greater as depart from fovea. No vision in 

 blind spot, nasal side, 2.5 mm from center of eye, 15 mm in diameter. 



Persistence of vision as related to color and intensjty is measured by increasing speed 

 of rotating sector until flicker disappears : for color, 0.4/i, 0.031 sec ; 0.45ft, 0.020 sec ; 0.5^t, 

 0.015 sec; 0.57/*, 0.012 sec; 0.68m, 0.014 sec; 0.76m, 0.018 sec; for intensity, 0.06 meter- 

 candle, 0.028 sec; 1 mc, 0.020 sec; 6 mc, 0.014 sec; 100 mc, 0.010 sec; 142 mc, 0.007 sec. 



Sensibility to small differences in color has two pronounced maxima (in yellow and 

 green) and two slight ones (extreme blue, extreme red). The sensibility to small differ- 

 ences in intensity is nearly independent of the intensity (Fechner's law) as indicated by the 

 following data due to Konig: 



1,000,- 



I/h 000 100,000 10,000 



dl/I, white 036 .019 .018 



.60 n — .024 .016 



.50 /i — — .018 



.43 n — — — .018 .025 .027 .040 .049 .074 .137 



TABLE 65.— DISTRIBUTION COEFFICIENTS FOR EQUAL-ENERGY STIMULUS 

 1931 I.C.I, standard observer 28 



The fact that almost any color can be produced by the proper mixture of 

 red, green, and blue light, has been used as a basis of a system of color specifi- 

 cations that has been adopted by the International Commission on Illumination. 

 In the system adopted by that Commission in 1931, the primaries are called the 

 X, Y , and Z stimuli. The properties of the standard observer are given by his 

 tristimulus specifications of the spectrum stimuli as a function of wavelength. 

 This table gives this specification for the equal energy spectrum. 



a'Judd, D. B., Journ. Opt. Soc. Amer., vol. 23, p. 359, 1933. 



(continued) 

 SMITHSONIAN PHYSICAL TABLES 



